A hydraulic excavator used at a mine etc. operates in the following manner: When a dump truck is stopped near the hydraulic excavator, the excavator scoops mineral ore or earth/sand from an excavation spot with a bucket thereof, swings by means of a revolving superstructure thereof to move the bucket to a position above the vessel of the dump truck, and then releases and loads the scooped mineral ore or earth/sand onto the vessel. Then, to again load the dump truck with mineral ore or earth/sand, the hydraulic excavator swings toward the excavation spot by means of the revolving superstructure. The hydraulic excavator repeats the above sequence of operations several times to load the vessel of the dump truck with a predetermined amount of mineral ore or earth/sand. After the loading is completed, the dump truck transports the mineral ore or earth/sand to a prespecified location.
While the loading is performed, a mine supervisor or the like needs to measure the amounts of loads placed on dump trucks by the hydraulic excavator, in order to manage the output of the mineral ore or earth/sand. The task is easy if dump trucks are equipped with movable load meters, but many of dump trucks do not have such load meters. Also, it is difficult to estimate the amount of load from the capacity of the vessel, because the vessel capacity varies depending on the types of dump trucks. Thus, with dump trucks alone, it is not easy to measure loads placed thereon.
Conventionally, when measuring the amount of mineral ore or earth/sand loaded onto a dump truck by a hydraulic excavator, the operator of the excavator operates a measurement button immediately before the mineral ore or earth/sand is loaded onto the dump truck, to measure the weight of the mineral ore or earth/sand.
There have also been proposed techniques for automatically measuring the weight of mineral ore or earth/sand scooped in the bucket. For example, Unexamined Japanese Patent Publication No. H06-10378 discloses a technique whereby a load in the bucket detected before the bucket is operated or swung is regarded as the load amount. Unexamined Japanese Patent Publication No. H07-259137 discloses a technique whereby a load in the bucket detected when the swing speed of the arm is lower than a fixed speed and at the same time the swing speed of the boom is higher than a fixed speed is regarded as the load amount. Also, Unexamined Japanese Patent Publication No. S62-274223 discloses a technique whereby cylinder oil pressures detected before and after the bucket reaches a predetermined height are averaged and a load in the bucket calculated from the average oil pressure is regarded as the load amount.
However, the aforementioned conventional method and the techniques disclosed in the publications are associated with problems. Namely, even in cases where the load in the bucket is unstable because of accelerating/decelerating operation of the boom or the arm or due to other factors, the load in the bucket is measured automatically or in response to operation of the measurement button, with the result that the amount of mineral ore or earth/sand loaded onto a dump truck cannot be measured with accuracy.
Also, the techniques disclosed in Unexamined Japanese Patent Publication No. H07-259137 and Unexamined Japanese Patent Publication No. S62-274223 are associated with a problem that the measurement itself cannot be achieved when predetermined conditions (predetermined swing speed, predetermined bucket height) are not fulfilled.
If the amount of mineral ore or earth/sand loaded onto dump trucks cannot be measured with accuracy, then it is not possible to accurately manage the output of the mineral ore or earth/sand.